Fluctuating asymmetry in insecticide-resistant and insecticide-susceptible strains of the maize weevil, Sitophilus zeamais (Coleoptera: Curculionidae)

Abstract

Insecticide resistance is an evolutionary response of specific insect populations subjected to exposure and consequent selection by an insecticide. As such, this phenomenon is important as a biomonitoring strategy and also has economical importance in the case of insect-pests by compromising their control. Fluctuating asymmetry (FA) of bilateral traits is a measure of developmental instability also suggested as a monitoring tool for environmental pollution with potential consequences for fitness. Responses to selective agents might have pleiotropic effects influencing development and pheno- type, which has yet to be examined for agricultural insecticides and insect-pests. Higher levels of FA are expected in the insecticide-resistant strains, which are usually at a selective disadvantage, relative to susceptible strains, in the absence of the insecticide. Two insecticide-resistant strains and an insecticide-susceptible strain of the maize pest insect Sitophi- lus zeamais (Coleoptera: Curculionidae) were subjected to FA measurements in 12 traits of wing veins and tibias of 100 individuals of both sexes of each strain. The insecticide- resistant strains showed lower FA than the susceptible strain, in contrast with the initial expectation. An extended period of insecticide selection probably led to the evolution of fitness- modifier genes improving the performance of the resistant genotypes, reducing their FA levels, and leading to their eventual fixation in the population. In addition, one insecti- cide-resistant strain and the insecticide-susceptible strain showed significant differences in FA between sexes, with more symmetrical males suggesting possible sexual selection by the females. The observed results have potential consequences for insecticide-resistance evolution and dispersal.